This invention relates to the selection of radio transmission resources in for example a transceiver station of a wireless cellular communications network.
The area covered by a cellular network is divided into a plurality of cells or cell sectors. Each cell is served by a base station which transmits signals to and receives signals from mobile stations located in the cell or cell sector. Base transceiver stations of cellular communications networks may have two or more antenna elements mounted at a certain distance from one another. Such arrangements exploit space diversity and reduce the effects of multipath fading. A downlink transmission technique suitable for use with transceiver stations having two or more antenna elements can involve periodically switching transmission from one antenna element to another according to a predetermined scheme. These types of transmission techniques are referred to herein as xe2x80x9ctime switched transmit diversityxe2x80x9d (TSTD) schemes. Time switched transmit diversity schemes are potentially useful in third generation mobile systems such as wide band code division multiple access (WCDMA) systems, for example systems operating to the Japanese ARIB specification, because they offer good performance with minimal implementation complexity.
Each transmitting antenna element of a transceiver station is connected to circuitry which is sensitive to the electrical loading conditions resulting from the antenna selection scheme. For example, in many transceiver station configurations each transmitting antenna element is connected to a power amplifier. The distribution of power across the power amplifiers depends upon the number of users (e.g. mobile stations) connected to each transmission branch. Power loading is a critical issue because the loading conditions dictate characteristics which the power amplifiers are required to possess. When selecting a power amplifier for a given application it is necessary to consider the mean power requirements and also the maximum amount of power (peak power) the amplifier will be required to withstand. Peak power loading results when a given transmission branch is selected to serve a large number of users simultaneously. It is necessary to select power amplifiers designed with correspondingly high crest factors CF (CF=Peak Power/Average Power). This design constraint can be relaxed if the electrical load can be evenly distributed between the available power amplifiers, thereby avoiding problematic power imbalances or high peak loads in any particular branch.
FIGS. 1a and 1b illustrate two different scenarios which have implications for electrical load balancing in multi-antenna systems. FIG. 1a shows a situation in which a given user transmits in successive time slots and the respective time slots of the different users #1 to #4 are synchronised in time. Corresponding time slots of the plurality of connected users have common start and end times. FIG. 1b shows an alternative situation in which the respective users operate on an asynchronous basis. In this case, corresponding time slots of the respective users do not share common start and end times. According to both the scenarios of FIG. 1a and FIG. 1b, if the mean electrical load is evenly distributed, the mean power supplied by each power amplifier of a two antenna system is equal to KP/2, where P is the power transmitted to one user and K is the number of users.
It will be appreciated that the synchronous case can be particularly problematic because a given power amplifier loading condition lasts for the entire duration of the time slot (or transmission burst). Frames in wide band code division multiple access systems are not necessarily xe2x80x9csynchronisedxe2x80x9d because a time slot system such as that in GSM is not employed. However a similar condition can be envisaged with a given power loading being maintained for the duration of a given power amplifier selection condition. Power amplifier selection conditions such as those seen with the frames of wide band code division multiple access systems tend in practice to be shorter than conventional GSM time slots.
According to an aspect of the present invention there is provided a method of selecting radio transmission resources of a first station for communicating between said first station and a plurality of second stations in a wireless communications network, the method comprising the steps of grouping said plurality of second stations based on a traffic characteristic of respective ones thereof and selecting at said first station radio transmission resources for communication with said second stations such that said transmission resources are used substantially equally within each said group.
Preferably, the plurality of second stations are grouped based on a characteristic indicative of the relative power requirements thereof. For example, the data transfer rate characteristic is a useful indication of the relative power requirements of mobile stations and the stations may be grouped such that all stations which belong to a given group operate at substantially equal data transfer rates.
Preferably, predetermined rules are applied within each of the defined groups to manage the selection of transmission resources at the first station.
In certain embodiments, the radio transmission resources selected from comprise two or more transmission branches for transmission from said first station to said second stations wherein transmission schemes employed switch between respective transmission branches according to a predetermined pattern. Typically, each of said transmission branches is connected to antenna switching circuitry. Each of the transmission branches comprises an antenna element. This arrangement can be found in, for example, multi-antennae transmission arrangements exploiting space diversity. That is systems in which antennae are separated in space to allow the use of diversity such that the radio channels associated with each antenna are non-correlated. Each of the transmission branches comprises a power amplifier for supplying signal power to the antenna of the same transmission branch under the control of the antenna switching circuitry.
Preferably, the selection of transmission resources is controlled within each group by making an initial antenna selection at said first station for each new second station establishing a link based on at least one previous event influencing the balance of power between said transmission branches. For Example, whenever one of said plurality of second stations establishes a new link and the preceding event which occurred in the relevant group was the establishment of a link, a different initial antenna selection is made to that which was made when the preceding link was established. Likewise, whenever one of said plurality of second stations establishes a link and the preceding event which occurred in the relevant group was the termination of a link, the initial antenna selection for the new link is made such that it is the same as the initial antenna selection made in respect of the previously terminated link.
More sophisticated embodiments ensure that within each group the initial antenna selection is made for a new link based on the loading conditions of the transmission branches at that time. For example, a counter associated with each group monitors the loading conditions of the transmission branches for that group. A count of zero indicates substantially balanced loading conditions for the group in question. Accordingly, whenever one of said plurality of second stations establishes a link and the count resulting from the preceding event in the group is less than zero, the first antenna is selected and the count is incremented by one. Whenever one of said plurality of second stations establishes a link and the count resulting from the preceding event which occurred in the relevant group is greater than zero, the second antenna is selected and the count is decremented by one. Whenever one of said plurality of second stations establishes a link and the count resulting from the preceding event which occurred in the relevant group is equal to zero, the first antenna is selected as the initial antenna and the count is incremented by one or the second antenna is selected as the initial antenna and the counter is decremented by one. In such embodiments, the count is incremented by one each time one of said plurality of second stations terminates a link for which the second antenna was the initial antenna selection. Similarly, the count is decremented by one each time one of said plurality of second stations terminates a link for which the first antenna was the initial antenna selection.
Preferably, the method is used to select between first and second transmission antennae in a system employing diversity transmission.
In other embodiments, the radio transmission resources selected implement the direction of transmission in a system employing spatial division of radio channels by means of one or more phased antenna arrays.
According to another aspect of the present invention, there is provided a method of selecting radio transmission resources of a first station for communicating between said first station and a plurality of second stations in a wireless communications network, the method comprising the steps of grouping said second stations into a plurality of groups based on a traffic characteristic of respective ones thereof and managing the selection of said transmission resources at said first station such that said transmission resources are used substantially equally by said second stations within each said group.
According to another aspect of the present invention, there is provided a method for selecting radio transmission resources of a first station for communicating between said first station and a plurality of second stations in a wireless communications network, the method comprising the steps of grouping said plurality of second stations based on a traffic characteristic of respective ones thereof and selecting at said first station radio transmission resources based on at least one previous event occurring within a group.
According to another aspect of the present invention, there is provided a method of selecting radio transmission resources of a first station for communicating between said first station and a plurality of second stations in a wireless communications network, the method comprising the steps of grouping said plurality of second stations based on a traffic characteristic of respective ones thereof and selecting at said first station radio transmission resources based on current usage of said transmission resources by said second stations within each said group.
According to another aspect of the present invention, there is provided a transceiver comprising circuitry for selecting radio transmission resources of said transceiver for communicating between said transceiver and a plurality of second stations in a wireless communications network, the transceiver comprising means for determining groups of said plurality of second stations based on a traffic characteristic of said second stations and means for selecting at the first station radio transmission resources for communication with said second stations such that said transmission resources are used substantially equally within each said group.
According to another aspect of the present invention there is provided a method of selecting between first and second transmission antenna elements of a first station for communicating between said first station and a plurality of second stations in a wireless communications network employing diversity transmission, the method comprising the steps of grouping said plurality of second stations based on a traffic characteristic of respective ones thereof and selecting at said first station an antenna element for communication with said second stations such that said transmission antennae are used substantially equally within each said group.
According to another aspect of the present invention there is provided a method of selecting between first and second directions available for transmission from a first station to a plurality of second stations in a wireless communications network, the method comprising the steps of grouping said plurality of second stations based on a traffic characteristic of respective ones thereof and selecting at said first station a transmission direction for communication with said second stations such that each of said transmission directions are used substantially equally within each said group.
The present invention seeks to provide improved methods and systems for use in the selection of radio transmission resources.